Pergamon Biomass and Bioenergy Vol. 13, Nos. l/2, pp. 63-73, 1997

c, 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: SO961-9534(97)00025-l 0961-9534/97 $17.00 + 0.00

ENERGY, EXERGY AND EMERGY ANALYSIS OF USING STRAW AS FUEL IN DISTRICT HEATING PLANTS

DANIEL NILSSON

Department of Agricultural Engineering, Swedish University of Agricultural Sciences, P.O. Box 7033. S-750 07, Uppsala, Sweden

(Received 26 October 1995: reoised 7 April 1997: accepted 5 Mav 1997)

Abstract-Straw is a renewable biomass with considerable potential as a fuel in most countries with cereal production. The harvesting, handling and conversion systems, however, require inputs of fossil fuels and other natural resources. In this study, straw is evaluated as a fuel in district heating plants with respect to energy requirements, exergy consumption and from an emergy point of view. Assuming straw to be a by-product of cultivation of cereals, the calculations show that the energy balance is favourable (12 : I) when direct and indirect energy requirements are taken into account (nitrogen replacement not considered). The exergy analysis, however, shows that the conversion step is ineffective in the sense that energy quality i\ lost. The emergy analysis, which is a method that tries to integrate the human economy with the ecological system, indicates that large amounts of energy have been used in the past to form the straw fuel (the net emergy yield ratio is 1.1). 0 1997 Elsevier Science Ltd

Keywords-Energy analysis; exergy analysis; emergy analysis; biofuels; bioenergy; straw fuels

i B

: d e E EIR F g g h H i I k

I L m

kc” NEYR P

6

;; S

I, Tr U

; Y z

1. NOMENCLATI’RE

Albedo [ - ] Area [ml] Emergy [sej] Capacity [tonnes h-‘1 Concentration [ppm] Eddy diffusion coefficient [m” se’] Specific (embodied) energy [J kg-‘] Energy [Jl Emergy investment ratio ( - ] Emergy from the economy [sej] Acceleration of gravity [m s-‘1 Gibb’s free energy, specific [J kg-‘] Average elevation [m] Height of atmospheric boundary [m] Average insolation (J m-‘) Emergy from the environment [sej] Correction factor for idle time, maintenance, etc. [ -1 Working life [h] Enthalpy of phase change [J kg-‘] Mass [kg] Substance amount [mole] Net calorific value [J kg-‘, J I-‘] Net emergy yield ratio [ - ] Pressure [Pa] Rainfall [m] Amount of heat [J] Fraction of rain runoff [ - ] Gas constant [J mole-’ Km’] Entropy [J Km’] Time [s] Temperature [K] Transformity [sej J-‘, sej g-‘1 Internal energy [J] Vertical gradient of wind [m s-’ mm’] Volume [m’] Yield [sej, tonnes ha-‘] Mass fraction [ - ]

7 Efficiency [ - ] P Chemical potential [J mole-‘] r Specific exergy [J kg-‘] E Exergy [Jl P Density [kg mm’] 4 Mass fraction of a material [ - ]

Subscripts

0 Reference state ch Chemical em Embodied eq Equilibrium W Water

2. INTRODUCTION

Energy analysis has attracted increasing attention during recent decades, especially after the first “oil crisis” in 1973. Initially, a num- ber of different methodologies and confusing nomenclature were used. This led to an international workshop in 1974, where the general outlines of energy analysis were laid down.’ The participants agreed on a general framework, which included conventions, pro- cedural aspects and terminology. It seems, however, that this was not comprehensive enough, because additional energy-related evaluation methods have come into use (e.g. exergy and emergy analysis).

Traditional energy analysis has been criti-

63

64 DANIEL NILSSON

cized from many points of view. The criticism concerns the suitability of using energy alone as a measure for resource use, as well as the fact that energy is not an unambiguous con- cept in the sense that different energy forms can be totalled. Also, the statement that all processes transform energy is self-evident according to fundamental thermodynamics. The main consideration is whether the energy obtained is useful or not.

IFIAS defined energy analysis as “the determination of the energy sequestered in the process of making a good or service within the framework of an agreed set of conventions or applying the information so obtained”.’ The concept has broadened since then, how- ever, as energy quality aspects have become more significant in recent years. IFIAS pro- posed Gibb’s free energy as a measure of quality.’ Gibb’s free energy is referred to a standard state and can be suitable for chemical reactions. However, a more general concept that refers to an environmental state is necessary if macro-systems (e.g. economic, technical or ecological systems) are to be investigated. The exergy analysis has emerged as a method of estimating the energy quality losses not only in physical/chemical processes, but also in such open systems.2

The interactions between energy analysis and economy have been discussed by a large number of authors. Georgescu-Roegen made an early link-up between economy and thermodynamics, especially with the concept of entropy.-’ He tried to integrate energetics, physics and economy. Furthermore, the interface between economy and ecology has been analysed by some biophysicists,4, ’ who argue that today’s econ- omic system does not sufficiently reflect the scarcity of natural resources. The holistic emergy model developed by Odum,6 for example, is an attempt to bridge the gap between the economic and ecological systems. He proposes a new measure of value, namely emergy, which describes the total amount of energy of one type required to form a unit of another type. On a more pragmatic level there are, on the other hand, scientists who claim that there are no evident connections between energy and economy for particular technical systems.7

Straw is a renewable biomass with consider- able potential as a fuel in most countries with cereal production. For sustainable use of biofuels, however, the net energy yield must

inevitably be positive in the long term. Previous investigations have shown that straw is a beneficial fuel; Swedish and Danish studies, for example, point out that the energy ratio (output : input) is well above 1.0 for using the straw in district heating plants,* as well as in combined heat and power plants9,” A study by Jenkins and Knutson” indicates that straw may be more questionable as a fuel in power stations.

Straw is obviously diluted and fairly value- less from an energy point of view when located in the fields. When harvested, high quality non- renewable fuels, such as diesel, are exchanged for energy in the form of refined, concentrated straw and eventually hot water. The aim of this study was to investigate the possibilities of enhancing the efficiency of such natural re- source use. Several indices have been calculated by using the energy (process), exergy and emergy approaches, in order to find out the feasibility of using straw as a fuel in district heating plants.

3. ENERGY (PROCESS) ANALYSIS

3.1. Methodology

The process analysis method is used here to calculate the energy balance of producing hot water in straw-fired district heating plants. Generally, the process analysis is used to estimate the energy requirements for production of goods and services, and to investigate the potentials to reduce energy costs. The direct energy used for successive production steps is traced backwards from the product to the primary sources (Fig. 1). The back system boundary is drawn at the level where the additional inputs are expected to be negligible. The applicability of process analysis may be limited by lack of data and where by-products and complicated reciprocal processes are involved.

The treatment of sunlight and labour in agricultural energy analysis is a disputed topic. When different technologies are to be com- pared, in principle it is not relevant to include sunlight because it will be the same regardless of the machines. An important question is whether labour should be seen as a free resource or not, and how, for example, should education and knowledge be valued in terms of energy. The energy input of human labour can be considered by measuring the

Energy. exergy and emergy analysis of using straw as fuel in district heating plants 65

Primary fuels

I I 1 I I I I I 1 I

I I I I

I I I I I I I ,

Machines for G Raw I I I material I material I_ Production ; , I I I of I I handling 1 I I ! 1 I & ! I I - I I 1 Product

I I Process I N I I 1-c

Machines I Machines 1 I to make + (capital I I

machines I goods) ; I 1 I I I I I I I

I I I I

Level 4 ; Level 3 ; Level 2 ; Level 1 I I I , I

Fig. 1. Outline of the process analysis method. Energy for transportation is included in the direct energy supply at every level.

expenditure of metabolic energy, measuring the “life-style support” energy requirement or measuring the marginal energy requirement of employment.7 Practically, the energy inputs of labour are often indicated as work hours.

3.2. Assumptions

Here, straw is regarded as a by-product and the calculations begin after the threshing of the crop. The straw is then followed until it is placed into the furnace and released as heat. Direct solar energy and energy embodied in manpower are not considered, as discussed above. The labour requirements are instead given as man-min tonne-’ delivered straw.

About 0.7 1 diesel tonne-’ straw is supposed to be saved when the straw is removed, because the combine harvester chopping and the possible soil incorporation will be excluded. Moreover, it is assumed that half of the acre- age is windrowed before baling. The straw is baled as high-density bales (weight 500 kg, dimensions 1.2 x 1.3 x 2.5 m’), which are col- lected and transported (3 km on average) to an intermediate store by tractor with front loader and wagon. The bales are stored in high stacks without cover. The storage losses are estimated to be 5%. Then the bales are transported (10 km on average) by truck to the heating plant (5 MW) and stored for a maximum of 3 days before firing.

The wagon loads 10 tonnes of straw, has an average speed of 15 km h-’ and requires 0.9 1 diesel fuel km-’ with a full load. The values for the truck are 16 tonnes, 50 km h-’ and 0.6 1 km-‘, respectively. The diesel fuel requirement for a round-trip is assumed to be

1.7 times the fuel requirement for a full load. The heating plant requires 280 MJ electric-

ity tonne-’ straw for its operation.’ It burns 8000 tonnes straw y-‘, and has a working life of 20 y. It is also assumed that it has two persons employed full-time on a yearly basis (2 x 2000 h), which means 30 man-min tonne-’ of straw delivered. The nutrients (except nitrogen) in the straw are returned to the fields via the ash. It is assumed that 0.3 1 diesel tonne-’ straw is required for this.

The total energy (per tonne straw) embodied in machines made of the materials j = 1 to m, used for the operations i = 1 to n, is estimated by

where m is the weight (kg), c the capacity (tonne straw h-‘) and 1 the working life (h) of the machine used in operation i, and 4, the mass fraction of material j (-) and e, the energy embodied in material j (J kg-‘).

Assuming that the harvesting and handling machines (excluding tractors and trucks) consist of 100% steel (4 = 1.00) and that erteel =41 MJ kg-’ (including maintenance and repair, estimated from Biirjesson”), the embodied energy in the machines will be Ee,mach = 36 MJ tonne-’ straw (see also Table 1). In a similar way, if the tractors have a working life of 10 000 h, weigh 7.0 and 4.3 tonnes, respectively, and consist of 60% steel and 40% iron (e,,,, = 27 MJ kg-‘),” the embodied energy will be E,,,,,, = 7 MJ tonnee’ straw.

DANIEL NILSSON

Finally, E,, truck = 1.2 MJ tonne-’ straw, if the truck is made of 70% steel and 30% iron.

The amount of indirect energy required for ash handling is small and can be neglected. The energy embodied in the heat plant is estimated to be 80 MJ tonne-’ straw,” including mainten- ance and repair during its working life. The energy required for producing diesel fuel12, ” and electricity (calculated in accordance with Ren- borg and Uhlin,13 by assuming that 50% arise from hydro-power and 50% from oil) is about 0.2 J JJ’ and 1.1 J J-‘, respectively.

3.3. Results

The direct energy (the net calorific value of diesel, NCVdi,,,‘, is 35.4 MJ l-l), indirect energy and labour requirements are totalled in Table 2. The total energy need to produce hot water will be 1.0 GJ tonne-’ delivered straw. With a conversion efficiency of 0.85, the energy balance will be 12 : 1 (with NCV,,,,, = 13.8 GJ tonne-‘). The energy balance will be 23 : 1 if indirect energy inputs are not considered. The direct labour input will be 61 man-min tonnee’ delivered straw or 5.2 man-min GJ-’ heat.

Straw contains about 6 kg nitrogen tonne-‘, which ought to be replaced in the long term. It is difficult, however, to estimate the amount of N fertilisers that should be taken into account in energy balance calculations. If all 6 kg are to be included, the indirect energy requirements will be increased by about 260 MJ tonnee’ straw, which reduces the total energy ratio to 9 : 1 (e, = 43.2 MJ kg-’ (Bertilssoni4)).

4. EXERGY ANALYSIS

4.1. The concept of exevgy

The concept of exergy is based mainly on the second law of thermodynamics. It can be used to locate and quantify losses of energy quality of processes, in order to improve the efficiency of energy use. The concept of exergy analysis and its theoretical basis was mainly developed and introduced in the middle of this century. The following definition was suggested by Baehr:” “ Exergy is that part of energy that is convertible into all other forms of energy”. Wall2 says that “energy is motion or ability to produce motion”, whereas “exergy is work or ability to produce work”.

Energy can only be converted into other forms by the reduction (consumption) of its quality. The driving force for all processes is the contrast to the environment or the disposition

Energy. exergy and emergy analysis of using straw as fuel in district heating plants

Table 2. Energy and labour requirements tonne-’ delivered straw (moisture content is 18%, wet basis)

67

Direct energy (MJ tonne-‘)

Indirect energy Labour (MJ tonne-‘) (man-min tonne- ‘)

Straw and ash handling 230 90 31 Plant operation 280 390 30 Total 510 480 61

to increase the entropy or disorder. Exergy is a the same assumptions and imaginary system property of the interactions between a system boundaries. However, the straw is only followed and its specified environment, which is in until it is placed in the furnace. The conversion internal thermodynamic equilibrium. The fol- step is instead analysed in greater detail in the lowing general formula expresses exergy (E) as next section. The handling system is assumed to the maximum amount of useful work that may be in thermal and mechanical equilibrium with be extracted from a system in its process of the reference environment, i.e. only the chemical reaching equilibrium with its environment’ exergy in the raw materials is considered.

= - I/ - or,., + po( v - &) I-

where the subscript eq refers to the equilibrium state and 0 to the state in the reference environment. The last summation term defines the chemical exergy in the system, where the difference of the chemical potential in the equilibrium and environmental states for sub- stance i are multiplied by the number of moles of that substance. Exergy associated with potential, kinetic and electric energy may also be taken into account in the expression above if they are present in the studied system. These

Straw is a varying material with a mixture of complicated organic and inorganic compounds. It is, therefore, difficult to calculate exactly the chemical exergy. The exergy of less complicated fuels can be estimated from tables of standard chemical exergy if the composition is known. The following equation was proposed by Szargut et al.,‘” and is based on regression equations; it is used here to express the specific chemical exergy of straw:

where

B= 1.041 + 0.216(z,,/z,.) - 0.250(2,,/2<)(1 + 0.788(+,/z<)) + O.O45(z,Jz,)

I - o.3o4(z,2/Z,.)

energy forms can be transformed into all other forms of energy, i.e. the quality index (the ratio of exergy to energy content) is by definition 1 .OO.

Straw is here evaluated as a fuel in district heating plants with respect to cumulative exergy consumption in the handling chain, and with respect to exergetic efficiency in the conversion step.

4.2. Cumulative exergy consumption

The cumulative exergy consumption describes the sum of all exergy in natural resources, energetic as well as non-energetic, that has been used in the production processes.‘h The system to be studied here is the same as that above, with

-

L denotes the enthalpy of phase change (J kg-‘) and z the mass fractions ( - ). Using the following data for moist straw; zc =

38.9%, zH2 = 4.9%, zO, = 33.8%, zN, = 0.4%, z,, = 18.0%, NCV = i3 800 kJ kg-‘, L, = 2440 kJ kg-’ and &,,,? = 50 kJ kg-‘, the specific chemical exergy will be 16 100 kJ kg-‘. In other words, the ratio of chemical exergy to the net calorific value is 1.16. (Note that the reference temperature is 25°C the relative humidity of atmospheric air is 70% and that the chemical exergy of ash is neglected.)16 The specific exergy of fossil fuels can be estimated in a similar way from data about their components. Hence, the ratio of <,,/NCV for diesel fuel will be 1.07 within the same reference system.

68 DANIEL NILSSON

The cumulative exergy consumption from the fields to the furnace will be (cf. Table 2)

1 .3 x 1.07 x 0.23 GJ,ri<,,\,,

+ 2.2 x 1.00 x 0.28 GJ,/,,.,,

+ 1.3 x 1.07 x 0.12 GJcrj<,\<,, = 1.1 GJ

tonne-’ straw, corresponding to an efficiency of 16.1/(16.1 + 1.1) = 94%. This ratio expresses the thermodynamic imperfection of the straw handling system, i.e. 6% of the exergy that is provided to the system is lost in the successive energy transformations. The factors 1.3 and 2.2 (MJ MJ-‘) describe the exergetic consumption for production of diesel fuel and electricity (50% from hydro-power and 50% from oil). In the third term, it is assumed that the energy required for production and repair of the machinery equipment and the plant is in the form of diesel fuel. The exergy input of steel and other materials can be neglected. Furthermore, the exergy for nitrogen replacement is not considered here.

4.3. Boiler eficiency

The exergetic efficiency of a heat plant can be evaluated as demonstrated in Fig. 2. It shows the system boundaries for a straw-fired furnace with an overall thermal efficiency of 85% (based on the NCV). The flue gases cross the boundary when they are at the same state as the environment and fully mixed with the air. The reference environment is specified at 5°C and 101.3 kPa. The external systems that interact with the furnace with respect to exergy are: supply of electricity and straw, and the steady flow of water that receives heat transfer. The following formula expresses the exergy transfer

to the water, corresponding to the conductive energy transfer:”

E WWJ -=l- z-7; Q

where the indices 1 and 2 represent the in- and outcoming water temperatures (K), and Q the amount of heat (J). Using the given values, the quality index (Z/Q) will be 0.206. If the exergy input from electricity corresponds to 2.0% of the NCV of straw, and the exergy input from the straw corresponds to 1.16 times its NCV (the ratio is practically the same as for the previous reference state), the exergetic efficiency of the boiler will be:

s5 x 0.206 = 14 80/ 116f2.0 ’ ’

This is a much lower figure than the energetic efficiency. The major exergy losses depend on the irreversible combustion process and the large temperature difference between the flame and the water.”

5. EMERGY ANALYSIS

5.1. Background and fundamentals

The emergy (abbreviation of “energy mem- ory” or “embodied energy”) analysis was developed by the systems ecologist Odum. A basic concept is transformity, which is defined as “the emergy required per unit product or service”.” The (solar) transformity is measured in solar emergy joules per joule or per gram of product (sej JJ’ or sej g-‘), and expresses the amount of solar equivalent energy that has been used in the past to form 1 g (or Joule) of that product. Therefore, the transformity is a

A Flue gases (p, To)

Straw (P,, T,) L

Air (P,. To) ) Water flow 95OC

Electricity ) Water return 60°C

Fig. 2. Analysis boundary for a straw-fired water boiler plant.

Energy, exergy and emergy analysis of using straw as fuel in district heating plants 69

measure of hierarchial position in energy transformation chains. The relationship be- tween the emergy (B,) of fuel i, for instance, and its transformity (TV,) is:

B, = Tr,E,

where E, denotes the energy content. The theoretical framework of emergy analysis is comprehensive and describes phenomena in both natural and human systems. However, a more practical approach applied on straw is used here to demonstrate the method. For more information about the fundamentals of emergy analysis, see references.h, I9 ?’

As illustrated in Fig. 3, two essential emergy indices can be derived. The net emergy yield ratio (NEYR) describes the emergy output of a process compared with the emergy inputs from the economy:

NEYR = Y/F

The emergy investment ratio (EIR);

EIR = F/I

expresses the degree to which emergy inputs from the economy are used to exploit environ- mental sources. Note also that

NEYR = 1 + (EZR)-’

for primary resource use, because Y = I + F.

5.2. Assumptions

An emergy diagram valid for 1 .O ha of cereal

cultivation is shown in Fig. 4. The field is assumed to be situated in south Sweden and the straw is harvested as high-density bales. The costs for goods and services are supposed to correspond to the revenues from sale of heat and grain. The transformity for goods and services is based on values from 1992 and assessed as:” (annual solar emergy use in Sweden)/(GNP) = 2.39.10z3 sejjl.4.10” SEK = 1.7.10” sej SEK-‘.

Straw is considered here to be a by-product of cultivation of cereals. All the emergy required for cereals production is, therefore, required for production of both grain and straw. The transformities for the products (grain and straw) will, however, be different because the energy content is different for each product. Emergy can also flow via so-called splits (see Fig. 4, in which splits are indicated by ramified arcs, whereas main (here grain) and by-product (here straw) pathways are indicated by separate arcs). The emergy of splits is divided in proportion to the flows, because the type and quality of the energy input is the same for each pathway. For example, the use of tractors in cereal cultivation and straw handling for a certain field is a split, because the same product (tractors) is involved for the same purpose (draught power).

The chemical potential in rain is chosen as the renewable input to the system, because its emergy contribution on a yearly basis is the largest. This is in accordance with Odum” in order to avoid double-counting. The environ- mental inputs for straw drying are considerable,

/ Fuels

/ Goods - Services

c Economic _ use

Y ) product

Fig. 3. Diagram showing the principles of emergy accounting. Note that emergy is not lost to the sink, just energy.

DANIEL NILSSON

I I

HaWeStiDgl handling

Fig. 4. An emergy flow diagram for straw fuels.

and very seldom taken into account in 5.3. Results conventional energy analysis. It is obvious that thousands of MJ tonne-’ dry matter are

The emergy in the heat (Bheat or Y& as

involved.” Compared with other annual en- indicated in Table 3) is calculated from

vironmental inputs, however, this contribution is of minor importance even if the straw was X6 + ,tna$4 Ii + 2 F,

rewetted again and again. It is not added in the ,=5

calculations because the environmental input where the index i represents the note numbers would then be double-counted. in Table 3. The solar transformity for the heat

Table 3. Resource evaluation table for the straw heating system (unit y-’ and ha-‘) (for notes see Appendix)

Basic data (J, tonnes, Solar transformity* (sej per Solar emergy Note Item SEK) unit) (IO” sej)

11: 1 2 3 4

FI: 5 6 7 8

F2: 9 10 11

Sunlight Wind, kinetic Rain, geopot.

Rain, chemical Diesel

Machines (steel) Nitrogen fertiliser

Goods and services Diesel

Machines (steel) Goods and

F3: 12 13

F4:

YI:

14

15

16

services Machines (steel)

Electricitv Goods and

services Goods and

services Heat

3~54.10’~ J I 8.72.10’” J 1496 3.43.10’ J 10 488 1.69.10”’ J 18 199 7.50.10’ J 66 000 l.70.104 g 1.9%109 1.20~105 g 4.62.109 6540 SEK 1.7.10”

35 130 36

308 495

34 554

1112

6.90.10” J 66 000 46 3.30.101 g l.98~10Y 7 1350 SEK 1.7~10” 229

6.56.10’ g l.98.109 1 8.40.10” J 200 000 168 3930 SEK 1.7.10” 668

300 SEK 1.7.10” 51

3.52~10”’ J Trhert 3673

*Transformities (excluding goods and services”) from Ulgiati et al.‘”

Energy, exergy and emergy analysis of using straw as fuel in district heating plants 71

(Trhea,) will be 1.0.105 sej JJ’ (Table 3). The net emergy yield ratio and the emergy investment ratio will be 1 .l and 11 for straw heat, respectively. It is, however, unsuitable to harvest straw every year in a field owing to losses of organic matter and top soil. These impacts depend, for instance, on the type of soil and are difficult to estimate. It may, therefore, be argued that the system is non-stationary, i.e. not sustainable. One possi- bility to overcome this is to assume that 4 ha of cereals are required for each ha on which straw is harvested. This would reduce the NEYR to slightly above 1 .O.

6. DISCUSSION

An energy balance of 12 : 1, valid from field to hot water, indicates that straw is a very profitable fuel from an energy point of view. The possibilities to further reduce the direct energy use for handling of high-density bales with present technology seems to be limited. The high-density baler works according to the normal pressure principle, where the bales are compressed by friction forces between the bale and the walls of the open bale chamber. This process is very energy demanding. Two alterna- tive handling methods have emerged in recent years. Tests with in-field chopping of straw and storing in uncovered stacks have been carried out in Denmark.23 The possibilities of enhancing the energy balance may be limited, however, owing to, e.g. high storage losses. A promising machine from an energy-saving perspective is the German compact-roller, which is under development. 24 It works, like round-balers, according to the radial pressure principle, which requires much less energy than the normal pressure balers.

The energy requirements for nitrogen replace- ment amount to fully 50% of the total direct energy requirements. If the straw is left in the fields, additional supply of nitrogen may be necessary to the following crop because of the high C/N ratio in straw, which leads to microbial binding of nitrogen in the decompo- sition process. In the long run, however, the nitrogen losses must be compensated if the straw is harvested. The difficulty in the energy accountings here is to estimate the amount of mineral nitrogen that is to be replaced by an N fertiliser.

The cumulative exergy consumption for

moving the straw from the fields to the plant is moderate. The cumulative consumption of chemical exergy in this system is strongly re- lated to the energy use, and for that reason the same conclusions that are discussed above are valid here. The exergy calculations for the conversion step show that boiler efficiency is low. McGovern has two proposals to increase the efficiency:” (1) extract the heat of combus- tion at the highest possible temperature by, e.g. interposing a power generating cycle; and (2) oxidize the fuel by a method with lesser irreversibility (exergy losses), e.g. by fuel cells. The latter method is hardly feasible today. The use of straw for electricity generation is limited due to low ash melting temperature and high temperature corrosion. There are, however, five straw-fired cogeneration plants in Denmark. They are all based on the Rankine cycle. To avoid slag deposits and corrosion in the steam/water tubes the superheater temperature is kept down, which in turn reduces the efficiency of electricity generation.

The emergy analysis shows that the NEYR is not more than 1.1, and that the EIR is as high as 11. In order to provide a perspective in this discussion we refer to Swartstrom, who presents emergy analyses for chunk-wood and oil used for heating purposes in 250 kW boilersI (see Table 4). As expected, the NEYR is largest for oil. One reason for these straw values is that the large resource inputs to the cultivation of cereals are attached entirely to the straw as it is seen as a by-product. About two-thirds of the emergy from the economic system is added in the cultivation of cereals (Fl in Fig. 4) and the NEYR is fully 1 .l already after this production step. These figures point out that present-day agriculture is based on large inputs of highly refined resources that have passed through the economic system. Note, however, that diesel fuel, steel, nitrogen and electricity are here considered as inputs both separately and included in goods and services. Calculating the inputs solely via money would, on the other hand, treat cheap resources with high emergy values unfairly. These issues hint that further

Table 4. Transformity, NEYR and EIR for heat from straw, chunk-wood and oil

Straw Chunk-wood Oil

Transformity (sej J-l) I .o.105 o.3.105 1.1.105 NEYR 1.1 1.5 2.6 EIR 11 1.9 0.6

72 DANIEL NILSSON

research about the application of the method is necessary.

7. CONCLUDING REMARKS ON THE METHODOLOGIES

The advocates of energy, exergy and emergy analysis often claim that their method is the most appropriate to measure energy quality. In energy analysis, energy quality can be deter- mined from the concept of EROI (energy return on investment). EROI is defined as “MJ of fuel extracted” divided by “MJ of direct and indirect energy required to locate, extract and refine that fuel”,4 and the higher the fuel quality, the higher the EROI. In exergy analysis, energy quality is defined as ability to do work in a physical sense.’ Finally, Odum states that the higher the transformities, the more emergy has been required to form that product and the higher is its value or quality.*’

The energy quality in EROI calculations is an attribute to the fuel, because 1 1 of oil can do the same amount of work independent of its quality in terms of EROI. Considering exergy, on the other hand, the quality of energy can be observed and measured in experiments. The exergy expresses the thermo- dynamic potential that a system possesses in relation to its environment. Emergy is the sum of all contributions of “useful work” in the hierarchial steps to produce goods or services. The quality of an item is here de- scribed as an inherent property that is related to its total production costs in terms of solar equivalent energy, which is quantified by network diagramming and transformity calculations.

The process analysis scheme is a prerequisite for assessing the net energy gain as well as the exergy losses in biofuel systems. The flows of materials and energy in the studied system are charted, and the different energy forms and qualities can be identified. The investigation can then be complemented by’ economic, emergy and life cycle assessments. The accuracy of the direct inputs to level 1 in Fig. 1 is, in most cases, satisfactory because data often are easily collected. The accuracy of indirect energy requirements may be more insufficient because the figures in the literature for energy embod- ied in production factors are calculated with specific assumptions, which may not be valid in the actual situation.

The emergy analysis considers not only

the environmental work, but also indirectly the human labour inputs. The accuracies of the transformities are, however, limited owing to lack of data and insufficient and inadequate knowledge about the studied system. Calcu- lations for technical systems on high hier- archial levels where the emergy is of the order of magnitude of IO” sej is, therefore, uncertain. The theoretical framework of emergy analy- sis has been criticised for using energy as a measure for “everything”,26 and for being built on unfounded hypotheses.27 However, the application here has indicated that the method’s strength primarily lie in its clarifica- tion of system structure and in its description of the environmental work that is provided to the system under consideration.

The energy-related evaluation methods used here have also been proposed to evaluate long-term sustainability in general. Sustainabil- ity is, however, a comprehensive concept that hardly can be evaluated with a single measure. Energy, exergy and emergy analyses can be used to evaluate the use of non-renewable and scarce natural resources in different production systems. On the other hand, it is difficult or even impossible to consider, for example, toxicity, eutrophication, acidification and ozone depletion with present calculation methods of these concepts. There is also no consensus on how such calculations would be performed and interpreted.

Energy, exergy and emergy analyses answer different questions, and the meaning of “energy quality” differs between them. It is, therefore, hardly productive to confront their justification as assessment tools, even though the results obtained in some cases may be hard to interpret and implement. Nevertheless, the application here has shown that they can all contribute to the understanding of the role of agricultural biofuels in the future energy systems.

Acknode&emmt.s-I am indebted to the Swedish National Board for Industrial and Technical Development for financial support. Many thanks also to Dr Anders Almquist for helpful discussions.

APPENDIX

8. FOOTNOTES TO Table 3

1 A,i.(l -a) = (10000 m’).(4600 MJ mm2).(1 - 0.23) = 3.54.10’2 J

Energy. exergy and emergy analysis of using straw as fuel in district heating plants 73

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6 1 8 9

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11

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(j,,;H.A.d.r’,r = (1.23 kg m-‘),(lOOO m).(lOOOO m’). assessments of combined heat and power plants- (25 m’ so ‘) ((3.0 m s’)/(lOOO m))‘(3.15 IO’s y-‘) energy and pollution analyses), dk-TEKNIK, Soborg = X.72.10’” J (1991).

p.A.(1;g.h = (0.70 m).(lOOOO m’).(lOOO kg mm’). (9.81 m ss’)(SO m) = 3.43.10’ J p (1 - r) A p* g, = (0.70 m) (0.49) (10 000 m’) (1000 kg mm’) (4.94 kJ kg-‘) = 1.69 IO’” J where R. = n R. T. log,(C,/C,) = (1000 g),i(l8 g mole-‘). (8.314 J Km’ mole-‘)(300 K)log,(999 990/965 000) = 4.94 kJ kg-’ (diesel requirements, including grain drying) NCVdlcr., = (180 kg (Sonesson”) (41.6 MJ kg-‘) = 750.10“ J

II.

12.

machinery requirements haa = 17.0 kg (Sonessor?) amount of nitrogen fertiliser in winter wheat = 120 kg total production cost = 6 540 SEK ha-’ (diesel requirements tonnee’ straw). r,,,;NCV,,,,,, = (6.5 I tonne-‘)(3.0 tonnes)(35.4 MJ I-‘) = 6.90.10” J (machinery requirements tonnee’ straw), X,,,,, = (I .I kg tonne-‘)(3.0 tonnes) = 3.30.10’ g (total production cost tonne-‘). E,,.,, = (450 SEK). (3.0 tonnes) = 1 350 SEK (weight steel in plant)/(lifetime),‘(tonnes straw y-‘). Y *I,.,* = (35 tonnes(Pedersen’“)):(20 y)/(8000 tonnes) (3.0 tonnes) = 6.56.10’ g (MJ el. tonne-’ straw). K:,,.,, = (280 MJ tonnee') (3.0 tonnes) = 8.40 x IO’ J (heat price) - (production costs) = ((0.15 SEK MJ ~‘). (0.85).(13 800 MJ tonne-‘).(3.0 tonnes)) - ((450 SEK tonnee’)(3.0 tonnes)) = 3930 SEK assume 300 SEK ha ’ Y \,nq NCV,,,,,, r~~,,~ = (3.0 tonnes).( 13 800 MJ tonne ‘). (0.85) = 3.52.10”’ J

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15.

16.

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18.

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